U.S. Pat. No. 5,233,911 discloses a typical tandem brake booster through which a force is developed to effect a brake application. In such a brake booster, corresponding first and second walls divide the interior into first and second front chambers and first and second rear chambers. The first front chamber is externally connected to a first source of fluid pressure by a conduit and internally directly connected to the second front chamber and to the first and second rear chambers by various passageways in a hub associated with the first and second walls. A control valve located in the hub is positioned such that the first fluid pressure is communicated in the first and second rear chambers through the passageways to define a rest or ready position. In responsive to an input force, the control valve is positioned such that a second fluid pressure is presented through a passageway to the first and second rear chambers to create a pressure differential across the first and second walls. This pressure differential acts on the first and second walls to develop an output force, which is provided to a master cylinder to assist in effecting a brake application. On termination of the input force, a return spring positions the control valve to a rest or ready location to define a first mode of operation where the second fluid is evacuated from the first and second rear chambers to provide for equalization of the first fluid pressure in the first and second rear chambers. This type vacuum brake booster functions in an adequate manner to provide an assist in effecting a brake application.
U.S. Pat. Nos. 4,409,885; 4,942,738; 5,313,796; 5,329,769 and 5,802,852 disclose brake booster and master cylinder arrangements wherein at least a portion of the master cylinder housing is positioned within the interior of the booster housing to define an integrated brake assembly. Such brake boosters operate in a satisfactory manner but because of their overall size occupies considerable under the hood space when installed in a vehicle. In this arrangement it is common for the brake boosters and master cylinders to manufactured in different locations and assembled as a central location. Unfortunately, the output push rod, which is part of the brake booster, must be separately shipped to the assembly location, as it is not always fixed to the movable wall.
In the vacuum brake boosters in the patents identified above a resulting output force produce in response to an output force follows a linear curve. In some applications it is desirable to modify the output force of a vacuum brake booster in a manner defined by the structure disclosure in U.S. Pat. No. 5,893,316 through a force transmitting assembly which includes a spring pack located between a reaction disc and plunger connected to an input push rod. In this vacuum brake booster, both the spring pack and push rod are separate items and separately installation in the vacuum brake booster before assembly on a vehicle. U.S. Pat. Nos. 4,892,027 and 4,898,073 disclose structure, which is designed to retain an output push rod within a brake booster.